I'm on an Italian forum where most people don't change the water and in small aquariums like those for a betta and some shrimp like caridina japonica they don't even put the filter. The aquariums seem to work great... to my great confusion
You raise excellent points. I think this point gives you the answer.
For Bettas and some species of shrimp, you can run no filter, no water change tanks. Both are evolved for swamp conditions. It isn't something I would do, but I have seen people be successful with it.
The fundamental concept here is the "species". We overlook it, but researchers see species as distinct for a reason. They all have different evolutionary histories, many of which stretch back long before our own. Conditions have shaped some species to thrive in conditions where others would die.
If you bear with me - water changes, and tiny tanks are a big debate in killifish world. I know breeders who use small aquariums and no filters, with minimal water changes, and who do better than I do. If I tried that with my soft tap water, it would be a disaster. Their water allows it - they tend to be grouped regionally. What works for you may not work for me, as water is chemically different in different regions.
I moved my fishroom 800 km, and had to relearn a lot. Species I had kept for years became difficult, and others I struggled with became easy. My fish were reacting to local water conditions with different minerals, and I had to learn how to manage that. I chose intensive water changing and have had good success with it. My water has few minerals, and tends to be chemically unstable. At my old house, the water was stable, buffered and the pH was close to impossible to change. I've realized how different water can be. Different water chemistry affects how we keep different species.
I like this approach - to myself, I say there is no such thing as a fish. Non esiste niente come il pesce...
There are species of the things we call fish, each of which has its own demands. Some are similar, if their habitats were similar. Some can look alike and have radically different aquarium needs. Get rid of the idea of fish and replace it with the idea of species. You, the fishkeeper have to adapt to the species, and then it isn't confusing. It is challenging, and it does call for learning.
I think you are confused because you are doing something very human. You are trying to see patterns and make broad rules to work by. Nature doesn't follow the limitations of the human brain. Learn about your water. Learn about fish species by species. Realize that different aquarists face different conditions. Realize that many aquarists are rigid and dogmatic, and like to fight with each other to establish dominance. Realize no one can tell you the correct way, and that you will have to choose a fishkeeping philosophy (mine is that water should be clean as a starting point, and I can experiment from there). Make a choice in the broad direction and work with it.
I grew up when water changing was seen as upsetting the natural aquarium's balance. That system failed miserably, in my experience, so I chose a different approach. It doesn't mean that for some species, that system won't work. But so will water changing, so I start with the safer approach.
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